The present invention relates to an information recording thin film, a method of its fabrication, and an information recording medium. More particularly, it concerns an information recording thin film or a superresolution readout thin film, a method of its fabrication, and an information recording medium having the information recording thin film or the superresolution readout thin film used therein that can record and reproduce information obtained by frequency-modulation of analog signals, such as video signal and audio signal, and digital information, such as computer data, facsimile signal, and digital audio signal in real time with use of energy beam, such as electron beam.
There have been many prior disclosures about principles of recording information in thin film, or recording film, by irradiation of laser beam. The known phase transition (or called the phase change) of film material, photo-darkening, and change of atomic arrangement by irradiation of laser beam do not deform the thin film virtually. They have the advantage that two disk members can be directly stuck together to obtain information recording media of double-side disc structure. The known recording film of GeSeTe system or InSbTe system has the advantage that information can be rewritten.
However, the recording films of the kinds mentioned are deteriorated in rewriting characteristics by flow of the recording film if recording is made as much times as 10.sup.5 at bit-position-recording or 10.sup.4 at bit-edge-recording. There have been proposed some methods to overcome such a disadvantage as the flow of the prior arts.
For example, the Japanese Patent Laid-Open No. 4-228127 disclosed a method of preventing the flow with the recording film made micro-cell. T. Ohta et al., "Optical Data Storage", '89 Proc. SPIE, 1078, 27 (1989), disclosed a method preventing the flow in the way that the recording film is made thinner to lower the heat capacity and makes use of increased effect of the adhesive force with the adjoining layer.
The usual optical discs can have analog information signals frequency-modulated with video signals or audio signals and digital information signals, such as computer data, facsimile signals, digital audio signals, transferred to surfaces of the substrate in the form of ruggedness. The usual optical discs also can record signals or data in real time with recording beams, such as laser beam and electron beam, on the recording thin film or the like.
The signal reproduction resolution of the usual optical discs is mostly determined in terms of a wavelength .lambda. of a light source of a reproduction optical system and an numeric aperture NA. The reading resolution is limited to a record mark interval of 2 NA/.lambda..
One of the techniques of high-density recording disclosed so far is the Japanese Patent Laid-Open No. 3-292632 in which a medium having the reflectance changed with phase change is used to reproduce the data recorded by ruggedness. Another technique is the Japanese Patent Laid-Open No. 5-73961 which has a medium of melting masking layer for reproducing the data recorded at a high density on a phase change recording film.
The term "phase changes" as used herein are construed to include not only a phase change between crystal and amorphous states, but also a phase change between melting which is a change to liquid phase and recrystallization and a phase change among crystal states as well.
The prior recording films for use as rewritable recording films of phase transition type are all involved in such problems as (1) not enough number of possible rewritings, (2) too slow crystallization speed with high number of possible rewritings, and (3) too low reproduced signal intensity with high number of possible rewritings.
Also, the technique disclosed in the Japanese Patent Laid-Open No. 3-292632 uses a Sb.sub.2 Se.sub.3 film on which phase change is partially made inside a scanning spot of reading light to change the reflectance so that phase bits only in areas of high reflectance can be read. This technique uses the film of a high melting point for which a laser irradiation power has to be too high to use for an optical disc of phase change type and magneto-optical disc other than the optical disc having information recorded by phase bit. The technique also has such a disadvantage that frequent readings cause the film to flow and segregate little by little, resulting in fewer number of possible superresolution readouts.
Further, the other technique disclosed in the Japanese Patent Laid-Open No. 5-73961 uses the medium of melting masking layer on which melting is partially made inside the scanning spot of reading light to change the reflectance, thereby making smaller apparent spot size. The medium used in the technique is the melting masking layer having low melting point and low viscosity. The technique also has the disadvantage that frequent readings cause the film to flow and segregate little by little, resulting in fewer number of possible superresolution readouts.